Preparations And Properties Of Ni-Pt Alloy Catalysts For Oxygen Reduction In Proton Exchange Membrane Fuel Cells

Conventional fuels are running out and polluting the environment seriously,so it is urgent to promote the development and transformation of green and low-carbon renewable resources.Proton Exchange Membrane Fuel Cell（PEMFC）is viewed as the most hopeful alternative energy source for green and efficient power transformation.However,the slow kinetic process of the cathode oxygen reduction reaction（ORR）restricts the total reaction speed.Platinum-based catalysts had became the most widely used ORR electrocatalysts due to its excellent catalytic activity.However,the durability of Ptis poor,the scarce content of Pton earth and the high price of Ptcause the high cost of PEMFC.Therefore,it is an important task for the current research on platinum-based catalysts to minimize the loading capacity of Ptwithout sacrificing the performance,and make reasonable use of the interaction between Ptand other materials to improve the catalytic activity and long-term stability of ORR.In this paper,the effects of octahedral Ni-Ptnanoalloy synthesized at different temperatures on its microstructure,chemical state and ORR catalytic properties were studied,and the growth mechanism and synergistic catalysis mechanism of the alloy particles were analyzed.On this basis,truncated octahedral Ni-Ptalloy particles doped with Se and I trace atoms were synthesized,and the mechanism of surface optimization was analyzed.The specific work are as follows:（1）The structure of the PtNi nanoalloy was truncated octahedral and the particle diameter was about 20nm.Due to the nano-size effect,more active sites were exposed,which made the catalytic activity of the active sites higher.Secondly,PtNi alloy nanoparticles had ideal（111）crystal plane and obvious Pt-rich edge of the polarization and dispersion structure,and Ni incorporation into Ptoccurred ligand effect and compression strain,which made the alloy particles had appropriate exposure surface and electronic structure.These important characteristics made the half-wave potential of PtNi truncated octahedral nanoparticles in acidic electrolyte was 74mV,which was higher than Pt/C,and the mass activity（MA）and specific activity（SA）were 5.1 and 5.7times that of Pt/C respectively.After 4 000 cycles,the mass activity remained as high as the 65.15%of the initial,showing good ORR durability.（2）The composition and morphology of the materials change with the synthesis temperature.With the increase of reaction temperature,the reduction rate of precursor Ni（acac）₂ increases,and the content of Ni increases,and the concave surface of incomplete growth gradually disappears.Therefore,the growth mechanism of PtNi truncated octahedron is a Pt-rich frame was formed first,and then filled the Ni-rich surface.In addition,the electrochemical tests showed that the alloy particles synthesized at 180℃ had the best catalytic properties.（3）Based on the synthetic method of the first work,we have successfully prepared PtNi alloy nanoparticles doped with trace Se and I atoms（Se-PtNi and I-PtNi）by in-situ synthesis and post treatment synthesis respectively.The degree of alloying was further increased,the Pt-Ptbond spacing was compressed,and the composition of Ni was slightly increased.The size of i-PtNi particles was smaller,which can exert the nano-size effect.However,in Se-PtNi,the segregation phenomenon was more obvious than that in PtNi octahedral nano-alloy,forming Pt-rich framework and Ni（111）rich crystal plane structure,which can effectively avoid excessive dissolution of Ni in acidic medium.On the other hand,Se atom can optimized the coordination environment of Pt,thus improving the ORR catalytic activity and stability of Se-PtNi.The electrochemical tests showed that the mass activity and specific activity of Se-PtNi alloy were 1.97A mg_Pt^-1 and 2.99mA cm^-2,which were 9.85 and 11.29 times of commercial Pt/C catalysts respectively.The ORR activity of Se-PtNi alloy was significantly improved.After 30000 cycles,the activity of Se-PtNi catalyst was still as high as 1.745A mg_Pt^-1 and2.87mA cm^-2,which only decreases by 11.1%and 4.14%compared with the initial mass activity and specific activity respectively.The stability of Se-PtNi catalyst is better than that of PtNi octahedron.